In order to acquire and process a biphase-coded signal such as the NAVSTAR/global position system (GPS) Coarse/Acquisition or Clear/Acquisition [C/A] code signal one needs to find the carrier frequency and the initial phase in the suppressed carrier signal received from a GPS satellite. The purpose of the acquisition system usually employed in a GPS receiver is to accomplish just these carrier frequency and initial phase determination functions in a signal, which actually has no carrier presence. Such an acquisition system in fact needs to perform a two-dimensional searching, i.e., searching in time and searching in frequency. This operation is time consuming; however, if one of the quantities is identified the other can be obtained rather easily because the search then becomes one-dimensional in nature.
A known signal processing method to determine the carrier frequency of a biphase-coded signal includes the step of squaring the frequency representation in order to remove the appended biphase code from the signal. Such a squaring operation in fact both doubles the frequency involved and eliminates the phase modulation component of the signal. In order to demonstrate this action a biphase coded signal, s(f), may be expressed mathematically in terms ofs=Asin(2πft+φ)  (1)where A represents the signal amplitude, f represents the signal carrier frequency and φrepresents the phase modulation impressed on the carrier (φ assumes values of +π and −π in representing biphase modulation). If a signal represented by equation 1 is processed by mathematical squaring the results are                               s          2                =                                            A              2                        ⁢                                          sin                2                            ⁡                              (                                                      2                    ⁢                    π                    ⁢                                                                                   ⁢                    ft                                    +                  ϕ                                )                                              =                                                                      -                                      A                    2                                                  2                            ⁡                              [                                  1                  -                                      cos                    ⁡                                          (                                                                        4                          ⁢                                                      π                            ⁢                            ft                                                                          +                                                  2                          ⁢                          ϕ                                                                    )                                                                      ]                                      =                                                            A                  2                                2                            [                              1                -                                  cos                  (                                      4                    ⁢                                          π                      ⁢                      ft                                                        )                                            ]                                                          (        2        )            In the final of the equation 2 three equalities the phase term, φ, has been eliminated through use of this squaring process. The carrier frequency of the received signal can be determined by way of fast Fourier transformation (FFT) processing of the squared signal representation; such Fourier transformation processing is a part of both the known signal processing and the present invention improvements.
In a real world environment with noise-inclusive signals this squaring process has the effect of increasing the noise component in the processed signal, especially under conditions where the noise is of greater magnitude than the signal and the bandwidth is relatively large. Therefore in order to find a signal a long record of data is often used. To perform Fourier transformation on a long data record is however complicated and time consuming. The present invention avoids these difficulties with a reduced Fourier transformation requirement.